I think if one were trying to find why successful players were being passed over in their first draft/going undrafted, it would be important to consider all of the variables, including obscurity. While it might not be feasible for a team to thoroughly scout every European junior team, it's still possible that one could try to find some sort of predictor of success at higher levels.
That said, I took a look through a few undrafted players and players who went undrafted their first eligible year to see where they played when they were passed over.
Undrafted: Drake Caggiula (OJHL/NCAA), Michal Kempny (Slovak juniors/Czech juniors/Czech Extraliga), Mats Zuccarello (Norwegian juniors/Norway), Torey Krug (USHL/NCAA), Tyler Johnson (OHL), David Desharnais (QMJHL)
Overagers: Tanner Pearson (GOJHL/OHL), David Perron (QJAAAHL), Wayne Simmonds (CJHL), Viktor Arvidsson (SuperElit/SHL)
Kempny and Zuccarello both played internationally, in addition to reaching their respective professional leagues while still draft eligible. Players drafted out of Norway are rare, but not entirely unheard of (Zuccarello did go to play in the SHL, but by then his draft eligibility had expired). Quite a few guys played Junior A, but I'm pretty sure outside of Caggiula the others were in leagues that don't get many, if any, drafted players. The other guys played in leagues that normally get players drafted (including Pearson in his second year of eligibility)
Obviously, though, such a listing is far from representative and skewed towards players who became good NHLers and not just depth/fringe players. That said, while looking over those players there is an obvious, and expected, correlation among many of them - they're undersized.